This invention relates to burners and is particularly concerned with burners which yield relatively low levels of nitrogen oxide (NO.sub.x) in their combustion products.
Nitrogen oxides (NO.sub.x) emitted from boiler and furnace plants, for example, have attracted considerable attention owing to the detrimental affect they have on the environment. Pulverised fuel, eg. coal or other like carbonaceous fuel, burners used in power generating stations are a major source of NO.sub.x. In such burners, NO.sub.x , emissions are generated both from atmospheric nitrogen (in dependence upon flame temperature) and from nitrogen fixed in the fuel (in dependence upon the amount of oxygen available during combustion).
An example of a pulverised fuel burner intended to reduce NO.sub.x emissions can be found in GB 2094969, where it is proposed to inject a swirling flow of air and fuel into supplementary air flow in order to combust the fuel in stages in sub-stoichiometric conditions. Similarly, in EP 160146 turbulence is created in the mixture of primary air and fuel by providing the outlet of the supply tube for that mixture with a flange of L-shaped cross-section, in effect a sharp edged nozzle, before combusting the fuel with secondary and tertiary air flows. More generally, known techniques for reducing the formation of NO.sub.x by pulverised fuel burners can be exemplified as follows:
by controlling the admission of air at the upstream end, relative to fuel/air flow, of the flame to avoid high flame temperatures thereby minimising the formation of NO.sub.x from atmospheric nitrogen;
by forming a fuel-rich region at the upstream end of the flame to release fuel nitrogen and other volatiles in the presence of sub-stoichiometric quantities of oxygen whereby the formation of NO.sub.x and of high temperature regions through the combustion of volatiles are minimised;
by maintaining the fuel-rich region so that any NO.sub.x formed in the early part of the combustion process can react with the fuel in a reducing environment to revert to nitrogen and carbon monoxide.
One way of establishing these conditions is to form a curtain of flame immediately around the edge of the fuel/air jet emerging from the burner. The purpose of this primary combustion stage is to create a flame in substoichiometric conditions that will provide heat to the fuel to release the fuel nitrogen and other volatiles. If secondary and tertiary air can then be added smoothly to the flow of fuel/primary air and volatiles without undue turbulence (which would cause high temperatures) it should be possible to achieve complete mixing and combustion within a volume similar to that occupied by a conventional high-turbulence flame.
The main difficulties in achieving these objectives are to ensure that a stable flame can be maintained at the fuel/primary air outlet from the burner, and then ensuring smooth mixing of fuel and air avoiding, on the one hand, excessive turbulence and hence high temperatures and NO.sub.x and, on the other hand, mixing that is delayed so long that it results in incomplete combustion of the fuel.